1. Field of the Invention
The present invention relates to a liquid crystal display device and to its fabricating method.
2. Background of the Related Art
Generally, a liquid crystal display device (hereinafter abbreviated LCD) includes a pair of glass substrates and an interposed liquid crystal that is sealed between those substrates. A TFT-LCD is an LCD device that includes thin film transistors (hereinafter abbreviated TFT) as switching elements that selectively applying signals across the liquid crystal. Thus, a TFT-LCD includes a TFT substrate with thin film transistors, a color filter substrate having color filters, and a liquid crystal between the TFT and color filter substrates. A TFT-LCD produces an image using the electro-optical characteristics of the liquid crystal.
Because TFT-LCDs have the benefits of low power consumption, lightweight, and portability, they hold the promise of being the next generation of imaging devices.
Reference will now be made in detail to an LCD according to the related art. Referring to FIG. 1, such an LCD includes a lower glass substrate 21, an upper glass substrate 21a, and a liquid crystal layer 23 between the substrates 21 and 21a. A plurality of crossing gate lines and data lines are formed on the lower glass substrate 21. A TFT is arranged near each intersection of the gate and data lines.
The TFT includes a gate electrode 25a on the lower glass substrate 21, a gate insulating layer 25b over the lower glass substrate 21 and the gate electrode 25a, and a semiconductor layer 25c on the gate insulating layer 25b and over the gate electrode 25a. The semiconductor layer acts as a TFT channel. A source electrode 25d and a drain electrode 25e are on the semiconductor layer 25c. A passivation layer 27 extends over the source electrode 25d, over the drain electrode 25e, over a portion of the semiconductor layer 25c, and over the gate insulating layer 25b. A pixel electrode 29 is disposed on the passivation layer 27 and in electrical contact with the drain electrode 25e. 
On the upper glass substrate 21 is a black matrix 31 arranged in a pattern to block light, except through the pixel electrode 29. The upper glass substrate 21 also includes color filters 33, which include red, green, and blue filters. A common electrode 35 is over the black matrix 31 and over the color filters 33. A voltage applied between the pixel electrode 29 and the common electrode produces an electric field in the liquid crystals 23. An overcoat layer (not shown in the drawings) may be added to protect and planarize surfaces before the common electrode 35 is formed.
FIG. 2A to FIG. 2E show cross-sectional views during fabrication of a related art LCD. Referring now to FIG. 2A, a metal layer is formed on a lower glass substrate 21 by depositing Al, Cr, Mo, Cu, Al alloy or the like, beneficially by sputtering. A gate line and a gate electrode 25a (see FIG. 2E) that extends from the gate line is formed from the metal layer by photolithography.
Then, a gate insulating layer 25b is formed on the lower glass substrate 21, including on the gate electrode 25a, using plasma enhanced chemical vapor deposition of silicon oxide (SiOx), silicon nitride (SiNx), or the like.
Referring now to FIG. 2B, a silicon layer is then formed on the gate insulating layer 25b by depositing amorphous silicon or the like. Then, a semiconductor layer 25c for the TFT channel is formed by patterning the silicon layer such that the silicon remains over the gate electrode 25a. 
Referring now to FIG. 2C, a metal layer is formed over the exposed surfaces, including over the semiconductor layer 25c, by depositing Al, Cr, Mo, Cu, an Al alloy, or the like by sputtering. Then, a data line that crosses the gate line, and source and drain electrodes 25d and 25e that extend from the data line are formed from the metal layer by photolithography.
Referring now to FIG. 2D, a passivation layer 27 is then formed over the exposed surfaces, including over the source and drain electrodes 25d and 25e. The passivation layer 27 is beneficially silicon nitride, silicon oxide, BCB or the like. A contact hole is then formed by selectively removing a portion of the passivation layer 27 to expose a portion of the drain electrode 25e. An ITO (indium tin oxide) layer 28 is then formed on an exposed surfaces, including in the contact hole, by sputtering.
Referring now to FIG. 2E, a pixel electrode 28a is then formed by removing a portion of the ITO layer 28 using photolithography, beneficially using an etchant belonging to the FeCl3 or the (HNO3+HCl) family.
After the TFT substrate is completed, that substrate is combined with a color filter substrate. Subsequently, a liquid crystal is interposed between the thin film transistor and the color filter substrates, thereby completing an LCD according to the related art.
Unfortunately, the related art LCD and its fabrication method has problems. The pixel electrode is patterned by etching an ITO layer using an FeCl3 or an (HNO3+HCl) family etchant. Such etchants can produce a metal oxide layer, or the pixel electrode pattern can be damaged, when a line (gate or data) metal of Al, Cu, or the like is contacted by the ITO etchant, which also contacts the ITO. Contact resistance can be increased, signals can be delayed, and electric shorts can result. Additionally, ITO is deposited in a sputtering device that includes very expensive vacuum equipment. Furthermore, the sputtering process temperature tends to be very high.